PROJECT SUMMARY/ABSTRACT Atherosclerosis is the primary cause of coronary heart disease, ischemic stroke, and peripheral arterial disease. This progressive vascular disease remains a leading cause of mortality in the United States despite wide-spread use of effective lipid-lowering therapies and prevention programs. While endothelial cell activation and neointimal hyperplasia is a hallmark of the initiation and progression of atherosclerosis, identification of molecular markers of dysfunctional endothelium represents a biological challenge as well as an opportunity to develop new therapeutic targets. In this application, we posit to explore the function of endothelial expressed CD45 in atherosclerosis. CD45 is best known as marker of hematopoietic cells. Our recent data shows that CD45 is indispensable in driving the endothelial-to-mesenchymal transition (EndMT) following myocardial infarction. It is thought that EndMT represents a novel endothelial perturbation and a critical regulator of atherogenesis given that EndMT disrupts the normal function of endothelium by delaminating endothelial cells and giving rise to neointimal mesenchymal cells, which secrete proinflammatory molecules and extracellular matrix proteins that fuel atherosclerosis progression. Whether endothelial CD45 contributes to progression of atherosclerosis by facilitating the EndMT is a completely unknown but highly significant question. In our latest studies, we showed CD45 null mice injected with the atherosclerosis accelerator PCSK9 AAv8 and fed a western diet had a marked reduction in atherogenesis. Conversely, we observed a significant increase in CD45 positive endothelial cells (ECs) that undergo the EndMT in atherosclerotic lesions. We also found that CRISPR/Cas9-mediated activation of CD45 in human ECs induces the EndMT. Intriguingly, CD45 expression is dramatically increased in HDAC2 knockout hemogenic endothelium, suggesting loss of HDAC2-mediated repression of CD45 during EndMT. Further, CD45 expression is altered in miR-155 deficient mice, indicating microRNAs may regulate the expression and function of CD45 during EndMT. Our pilot RNA seq study shows upregulation of epsins and downregulation of KLFs upon CD45 overexpression. Given that epsins promote and KLFs impede atherosclerosis, whether CD45 induces EndMT and promotes atherosclerosis by modulating epsin and KLF function is an entirely novel question. To test this, we will determine the role of endothelial CD45 in regulating atherosclerosis in vivo and interrogate molecular mechanisms 1) by which endothelial CD45 regulates EndMT in atherosclerosis by controlling epsin and KLF expression and 2) underlying how CD45 expression is epigenetically regulated in ECs during EndMT and whether targeting endothelial CD45 promotes atheroma resolution. Our preliminary data presents robust evidence that serves as a strong scientific premise for our proposed study. If successful, our study will not only provide extensive mechanistic evidence for a central role of CD45-mediated signaling in regulating the EndMT, but will provide an innovative approach for the treatment of atherosclerosis and launch a paradigm shift in research to combat cardiovascular disease.